Cognitive-Motor Interference During Dual-Tasking Among Healthy Adults and Chronic Stroke Survivors

Postural stability during dynamic postural tasks is achieved through an interaction between sensory systems, different movement strategies, and cognitive processing to attain the central goal of maintaining stability. Due to dynamic nature of the environment, events causing falls are unexpected and sudden. This places a substantial demand on an individual’s ability to attend to sudden changes in the environment to prevent a fall. The overall purpose of this dissertation to understand attentional demands locomotor and balance tasks using a dual-task paradigm among healthy adults and how presence of cerebral injury such as a stroke impacts attentional demands of postural tasks. Chapter 1 focuses on the interference between walking and different higher cognitive tasks focused on functions considered to play a role in locomotion. It also examines the effect of altering walking speed on the interference between walking and cognitive tasks. Healthy young adults walked at preferred and slow speeds while performing a visuomotor reaction time (VMRT), memory recall (word list generation), working memory (serial subtraction) and an executive function (Stroop) task. Effect of dual-tasking on walking and cognitive tasks was measured as the cost of dual-tasking for walking speed and cognitive performance. Results show that the motor and cognitive cost of dual-task walking depends heavily on the type and perceived complexity of cognitive task being performed. Cognitive costs for the Stroop task were low irrespective of walking speed, suggesting that at preferred-speed individuals prefer to prioritize complex cognitive tasks requiring higher processing resources over the walking. While performing VMRT, individuals preferred to prioritize more complex walking task over VMRT task resulting in lesser motor cost and increased cognitive cost for VMRT task. Chapter 2 compares the cognitive-motor interference (CMI) pattern of walking among chronic ambulatory stroke survivors and young adults to understand the effect of chronic stroke on dual-tasking function in comparison with individuals without any effect of aging or neurological condition. Community-dwelling chronic stroke survivors and young adults performed visuomotor (VMRT), serial subtraction (SS) and Stroop tasks while sitting and walking. Dual-task walking led to significant decline in motor and cognitive performance in both the groups. The stroke group showed highest motor cost for SS task, whereas young group showed highest motor cost for Stroop task. Although cognitive costs for both the groups were highest for VMRT and least for Stroop tasks, the cost for SS task was significantly greater among stroke survivors than young adults. The findings suggest that CMI pattern in chronic stroke survivors differs significantly with the type of cognitive task. Gradual cognitive decline with chronicity of the condition superimposed with aging might have a role in altering the CMI pattern post stroke. Chapter 3 examined the CMI of reactive balance control under dual-task condition in young healthy adults. Individuals were exposed to sudden large slip-like perturbations delivered in stance with (dual-task) and without a working memory task. This cognitive task was also performed in quiet stance. Dual-tasking significantly reduced postural stability and compensatory step length, and delayed the reaction time. The significant linear correlation between postural stability and compensatory step length present in the single-task balance condition, was absent in the dual-task condition. Cognitive task performance also declined under the dual-task condition. Our results indicate a mutual CMI pattern between the compensatory stepping responses to large perturbation and working memory tasks suggesting a potential overlap between attentional resources allocated for these two tasks. Chapter 4 focuses on differences in balance recovery mechanisms contributing to fall risk during large forward (inducing SLIPS) versus backward (inducing TRIPS) perturbations among healthy adults and chronic stroke survivors. Younger adults, age-matched older adults and chronic stroke survivors were exposed to a single SLIP and TRIP through a motorized treadmill. Center of mass (COM) state stability, trunk and compensatory step kinematics were recorded. The incidence of SLIP related falls among stroke survivors was higher than that in healthy (young and age-matched) adults however, not for TRIPS. All the groups showed higher stability change from liftoff to touchdown during TRIPS than SLIPS. Higher stability during TRIPS in healthy individuals was attributed to the ability to control trunk flexion at step touchdown and lower peak trunk velocity as compared with SLIPS. Chronic stroke survivors increased compensatory step length during TRIPS versus SLIPS contributing to greater stability change. Nevertheless, they were unable to control trunk excursion and velocity as compared with healthy adults leading to a lower stability than healthy younger and age-matched adults during SLIPS and lower stability than younger adults during TRIPS. Difficulty in trunk control during SLIPS among all individuals and compensatory stepping response among stroke survivors emphasizes higher fall risk for SLIPS than TRIPS among these populations. Lastly, Chapter 5 compares the CMI across different dynamic postural tasks and explores the effect of aging with and without a chronic stroke on the interference pattern. Young adults, age-matched older adults and older chronic stroke survivors performed an intentional balance task (limits of stability – LOS in forward direction), forward walking task, and forward compensatory stepping (reactive balance) tasks with and without a serial subtraction task. The maximum center of pressure excursion (MXE), gait velocity and COM position relative to base of support at step touchdown (XCOM/BOS), and correct responses on the cognitive task were recorded. The motor and cognitive cost of dual tasking were computed for the three postural tasks. Among healthy adults (younger and age-matched), higher motor cost was associated with the reactive balance and gait tasks with the least motor cost for the LOS task suggesting greater interference and therefore, higher attentional demands for postural tasks that are perceived more unstable. Among chronic stroke survivors, all the postural tasks demanded similar attentional resources seen by similar motor costs for all three postural tasks. Motor cost for all the tasks was lowest in younger adults. Gait and reactive balance motor costs were similar between age-matched adults and stroke survivors however the LOS cost was lower in age-matched adults thank stroke survivors. Although stroke survivors show disproportionate ability to divide attention between motor and cognitive tasks, in the chronic phase, impaired CMI could be related to both aging and stroke related sensorimotor deficits.